1. Field of the Invention
The present invention generally relates to video display systems. More particularly, the present invention relates to filtering of pixel data in a video display system. More particularly still, the invention relates to performing zoom operations on field-based images in a digital video system.
2. Background of the Invention
The consumer electronics industry has experienced a dramatic explosion in product development over the last 20 years. This explosion has been fueled by consumer demand coupled with significant advances in semiconductor technology that have lead to lower cost semiconductor devices incorporating significantly more functionality than previously possible. For example, a hand-held calculator from 20 years ago provided the ability to perform rudimentary mathematical operations. Today, a hand-held device can provide much of the functionality of a desktop computer system.
The visual display of information to the user is of particular importance to the consumer electronics industry. The most notable examples of visual displays include televisions and personal computers. Other types of consumer electronics, including stereo receivers and hand-held computers, also include visual displays. Typically, a visual display comprises a grid of "pixels" arranged in columns and rows. In a television format, the screen includes 720 columns and 480 rows of pixels.
Each pixel on the screen is represented by one or more data values that define the color of the pixel. Several standard formats are available for the pixel data values. For example, each pixel can be represented in a "RGB" format comprising a red color component, a green color component, and a blue component. Often, each red, green, and blue component is represented by an eight-bit value, thus requiring 24 bits to represent the entire RGB pixel value. Alternatively, each pixel can be represented in a "YUV" or "YCrCb" formats. In either the YUV or YCrCb format, the "Y" value represents luminance (or simply "luma") which is the brightness of an image. The U and V values represent chrominance (or simply "chroma") components and are calculated as the difference between the luminance components and the read and blue color values. That is, U=Y-R and V=Y-B. The Cr and Cb values also represent chrominance and are scaled U and V chrominance values.
The image displayed on a television monitor in each instance of time thus includes almost 350,000 pixels of information with each pixel represented by perhaps 24 bits (i.e. three bytes) of RGB or YCrCb values. In a television format, 30 frames of video are shown on the screen each seconds. Because of the extraordinary volume of data represented by motion video, compression techniques are important for the transmission and storage of video. One such compression technique is implemented by the MPEG standard ("Moving Pictures Experts Group").
The MPEG standard represents a set of methods for compression and decompression of full motion video images. MPEG compression uses both motion compensation and discrete cosine transform ("DCT") processes, among others, and can yield relatively high compression ratios. The YCrCb format for representing pixel color is the format specified by the MPEG standard.
The two predominant MPEG standards are referred to as MPEG-1 and MPEG-2. The MPEG-1 standard generally concerns inter-field data reduction using block-based motion in compensation prediction ("MCP"), which generally uses temporal differential pulse code modulation ("DCPM"). The MPEG-2 standard is similar to the MPEG-1 standard, but includes extensions to cover a wider range of applications, including interlaced digital video such as high definition television ("HDTV").
The MPEG format thus specifies various techniques for compressing motion video images. To display those images on a television or computer screen, the compressed images must be decompressed and then processed. The processing steps required after the images are decompressed includes one or more filtering steps. Video display systems, such as digital video disk (DVD) drives, usually include both horizontal and vertical filters. Horizontal filters process pixel data across a horizontal row of pixels. Vertical filters processed pixel data along a vertical column of pixels.
Video images may be represented in either a "frame" structure or a "field" structure format. In a frame structure, which is supported in the MPEG-1 standard, the entire 720.times.480 grid of pixels represents one instance in time. Thus, each 720.times.480 frame of pixels is captured at the same point in time. In a field structure format, each frames comprises two fields--an odd field and an even field. The odd field includes the odd numbered lines of pixels (i.e, lines 1, 3, 5, etc.) and the even field includes the even numbered lines (i.e., lines 2, 4, 6, etc.). The video system alternately displays the odd and even fields. Both fields thus are interlaced and displayed 30 times per second. Because these techniques are used for moving pictures, the odd fields are captured at slightly different points in time than the even fields.
It is often desirable to manipulate the size of a video image. For example, it may be desirable to zoom in on a portion of the video image. Accordingly, a processing technique is needed to increase the number of lines of pixels used to represent a portion of an image. One zoom technique that has been suggested involves simply copying pixels from the original image into adjacent pixel locations. Although fast, this technique results in inferior quality images. Thus, an improved zoom technique is needed.
Existing digital video systems generally implement various techniques for vertical filtering which generally requires combining or otherwise processing one line of pixel values with one or more other lines of pixel values. Such techniques generally and adequately process and entire frame of video data. Such vertical filtering techniques, however, are not sufficient to handle field structure images in which the odd lines of pixels were captured at a different point in time in the even lines of pixels. Because existing vertical filtering techniques do not take into account the time difference between portions of the image, such techniques thus effectively combine two distinct images representing different points in time. The quality of the resulting filtered image sufferers. The detrimental effect on the resulting image becomes more pronounced for images moving at higher speeds.
Thus, a video processing system is needed that is capable of performing zoom and other types of vertical processes on digital moving images. Further, such a technique should take into account the time difference between fields in a field-based video structure. Despite the advantages such a system would offer, today no such system is known to exist.